Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754075Ab3DVXOK (ORCPT ); Mon, 22 Apr 2013 19:14:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:25806 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752511Ab3DVXOI (ORCPT ); Mon, 22 Apr 2013 19:14:08 -0400 Message-ID: <5175C42C.1030901@redhat.com> Date: Mon, 22 Apr 2013 19:13:48 -0400 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130402 Thunderbird/17.0.5 MIME-Version: 1.0 To: Andi Kleen CC: Jiannan Ouyang , LKML , Raghavendra K T , Peter Zijlstra , Avi Kivity , Gleb Natapov , Ingo Molnar , Marcelo Tosatti , Srikar , "H. Peter Anvin" , "Nikunj A. Dadhania" , KVM , Thomas Gleixner , Chegu Vinod , "Andrew M. Theurer" , Srivatsa Vaddagiri , Andrew Jones , Karen Noel Subject: Re: Preemptable Ticket Spinlock References: <51745650.9050204@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 919 Lines: 24 On 04/22/2013 05:56 PM, Andi Kleen wrote: > Rik van Riel writes: >> >> If we always incremented the ticket number by 2 (instead of 1), then >> we could use the lower bit of the ticket number as the spinlock. > > Spinning on a single bit is very inefficient, as you need to do > try lock in a loop which is very unfriendly to the MESI state protocol. > It's much better to have at least three states and allow > spinning-while-reading-only. > > This is typically very visible on systems with >2S. Absolutely, the spinning should be read-only, until the CPU sees that the desired bit is clear. MESI-friendly spinning is essential. -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/